PSI - Issue 14

Priti Kotak Shah et al. / Procedia Structural Integrity 14 (2019) 242–250 Priti et. al. / Structural Integrity Procedia 00 (2018) 000–000

247

6

400

350

300

250

J (kJ/m 2 )

200

150

100

RT Test Axial notch Transverse notch

50

0

0

0.5

1

1.5

2

del a (mm)

Fig. 9. J vs a plot for axial and transverse notched specimens at room temperature.

A majority of the basal poles in the pressure tubes are oriented in the circumferential or transverse direction and to some extent, in the radial direction. With majority of { 0001 } poles aligned in the transverse direction, it requires the plastic flow to occur primarily by the more di ffi cult 101¯ 1 1¯ 1¯ 23 pyramidal slip (Bose and Klassen, 2011) or by twinning (Kim, 2008) when straining along transverse direction. Thus pyramidal slip and twinning is responsible for higher YS in the transverse specimen because the critical resolved shear stress(CRSS) for pyramidal slip and twinning are higher than that for prismatic slip. Once yielding occurs by twinning, further deformation occurs primarily in the twinned regions that are now better aligned for slip. The deformation will then take place locally there, leading to localized deformation in the necked region in transverse specimen. The axial specimens when loaded deform by slipping as the prismatic slip systems are better oriented in this orientation of specimen and the CRSS for this slip is lower than that for twinning. This leads to lower YS in the axial specimen of the Zr-2.5Nb pressure tube. Slip fosters uniform deformation along the gauge section and it leads to greater extent of strain hardening. As compared to the localized deformation in transverse specimen, the axial specimens show more uniform deformation along the gauge length. J vs a plots obtained from room temperature tests for axial and transverse notched specimens are shown in Figure 9. It shows that at room temperature the initiation fracture toughness parameter ( J i ) of axial notched specimen is relatively more than that of transverse notched specimen and the crack growth resistance parameter dJ da is more or less same for specimens of both orientations. However, at 300 0 C , dJ da is higher for axial notched specimen while J i is comparable for both type of specimens as seen in J vs a plot obtained in 300 0 C test in Figure 10. Plot of di ff erent fracture toughness parameters versus temperature for both types of specimens are shown in Fig ures 11-14. Initiation fracture toughness parameter has shown maximum scatter w.r.t temperature while scatter is relatively lower in other fracture toughness parameters. This may be due to the fact that it is di ffi cult to determine exact point of crack initiation while using DCPD for crack length measurement during the test. As-fabricated Zr-2.5Nb pressure tube material undergoes fully ductile fracture at all test temperatures. The fracture process is thus governed by micro-void nucleation, growth and coalescence mechanism due to intense plastic defor mation ahead of the crack tip. In the fracture toughness tests, when crack growth is along axial direction, this can be understood as breakage of small segments of material being pulled along transverse direction that is perpendicular to crack plane. Similarly, when crack grows in transverse direction it is axial tensile elements that undergo fracture at the crack tip. The fracture energy is expected to be function of total absorbed energy to fracture unit cells ahead of crack tip under triaxial loading. The transverse direction has higher strength, due to higher fraction of basal pole along this direction, but it has lower ductility. Therefore at ambient temperature J i and dJ da in both orientations are nearly similar. However, at higher test temperatures more slip systems come into play when loaded along transverse direction. There-